1. Field of the Invention
The present invention is directed to a pixel (picture element) structure of an active pixel sensor (APS), and in particular to a pixel structure of an active pixel sensor (APS) which can carry out a reset operation by pixel units by providing the pixels with a double reset structure to implement an electronic shutter function.
2. Description of the Background Art
Among pixel structures employing CMOS sensors, the most successful pixel structure in processing noise is a pixel structure using an active pixel sensor (APS). A remarkable characteristic of the pixel structure using the active pixel sensor (APS) is that each pixel amplifies the image information accumulated in a photo-diode, namely, the charge caused by photoelectrons, before converting it into a voltage.
FIG. 1 is a block diagram illustrating a conventional pixel structure using an active pixel sensor (APS). As shown in FIG. 1, the conventional pixel structure includes: a plurality of pixels P11-Pmn arrayed in a lattice pattern of rows and columns; a row selection decoder 1 selectively outputting a row selection signal ROW to the plurality of pixels P11-Pmn; a first line counter LC1 counting a number of lines of the pixels selected by the row selection decoder 1; a reset selection decoder 2 selectively outputting a reset signal RS to each row of pixels; a second line counter LC2 counting a number of lines of the pixels selected by the reset selection decoder 2; a column reading unit 3 reading image information stored in the pixels; and a pixel counter PC1 counting a number of the pixels read by the column reading unit 3.
FIG. 2 is a circuit diagram illustrating a pixel PXji located in a predetermined position. As shown in FIG. 2, the pixel PXji includes: a first NMOS transistor NM21 with its gate receiving the reset signal RS and its drain receiving a power supply voltage VDD; a photo-diode PD with its anode connected to ground and its cathode connected to the source of the first NMOS transistor NM21; a second NMOS transistor NM22 with its gate commonly connected to the source of the first NMOS transistor NM21 and the cathode of the photo-diode PD, and with its drain receiving the power supply voltage VDD; and a third NMOS transistor NM23 with its gate receiving the row selection signal ROW, its drain connected to the source of the second NMOS transistor NM22, and its source outputting an output signal OUT.
The operation of the above-described conventional pixel structure using the active pixel sensor (APS) will now be explained.
First, the reset signal RS outputted from the reset selection decoder 2 is inputted to the gate of the first NMOS transistor NM21 of a selected pixel PXji which is located in a predetermined position, and thus the first NMOS transistor NM21 is turned on. The first NMOS transistor NM21 discharges photoelectrons generated by incident light, thereby draining any internal electrical charge from the photo-diode PD. In this case, the reset operation is carried out in line units.
Then, when the photoelectrons are accumulated in the photo-diode PD by the incident light for a predetermined photosensing time, the voltage produced by accumulated photoelectrons is amplified by the second NMOS transistor NM22.
At this time, the row selection signal ROW outputted from the row selection decoder 1 is applied to the gate of the third NMOS transistor NM23, and thus the third NMOS transistor NM23 is turned on. Accordingly, the output signal OUT is outputted from the source of the third NMOS transistor NM23 to the column reading unit 3.
The output signal OUT represents the output image information of the selected pixel PXji.
Herein, the term xe2x80x9cinformationxe2x80x9d refers to the second image property, typically the light intensity, that is, the photonic energy incident on the photo-diode in each pixel sensor, which causes the accumulation of electrical charge (i.e., electron-hole pairs generated in a semiconductor junction) and in terms of an induced voltage potential due to photo-electrons producing a current.
However, the conventional pixel structure of the active pixel sensor (APS) has a disadvantage in that, as the reset operation is carried out not in pixel units but in line units, an electronic shutter function cannot be utilized.
Accordingly, it is an object of the present invention to provide a pixel structure using an active pixel sensor (APS) which can provide an electronic shutter function by carrying out a reset operation by pixel units.
In order to achieve the above-described object of the present invention, there is provided a pixel structure of an active pixel sensor (APS) with an electronic shutter function, including: a plurality of pixel sensors arrayed in rows and columns; a row selection decoder outputting a row selection signal ROW to each pixel sensor; a first line counter counting a number of lines of the pixels selected by the row selection decoder; a line reset selection decoder outputting a line reset signal to each pixel sensor; a second line counter counting a number of lines of the pixel selected by the line reset selection decoder; a column reset selection decoder outputting a column reset signal to each pixel sensor; a first pixel counter counting a number of the pixels selected by the column reset selection decoder; a column reading unit reading image information stored in the pixels; and a second pixel counter counting a number of the pixels which are read by the column reading unit.